Modern high speed wireless Local Area Networks (WLANs) are comprised of a number of individual devices. These can be either clients or access points, and they communicate with each other via wireless links. In order to use these wireless links, each device must have both a transmitter and a receiver. Furthermore, to achieve-optimal performance, the operating characteristics of the transmitting/receiving circuitry must be known. This requires the testing of each transmitter (and receiver) component prior to incorporation into a final WLAN product.
The clients and access points that make up a WLAN communicate with the other clients or access points over the wireless links using one of the many standardized protocols that have been developed for use in WLANs. Some of these protocols, such as 802.11a and g, make use of an OFDM modulation technique to encode the data to be transmitted onto a plurality of sub-carriers to convey the information to the receiver. Each sub-carrier is at a separate frequency that is orthogonal to, and equally spaced from, the other sub-carrier frequencies. An orthogonal frequency set is such that all frequencies other than the wanted frequency pass through zero at the wanted frequency. This ensures that the wanted frequency can be separated without the need for individual sub-band filters. It is the amplitude and phase of these individual sub-carriers which determine the data being carried by them.
In WLAN systems according to the 802.11a or g specifications, there are 52 individual sub-carriers, normally identified as −26 to −1 and +1 to +26.
In general, the data stream to be transmitted is split up into a plurality of data sub-streams, each sub-stream consisting of a first, lower data rate, and a second, higher data rate. The individual sub-streams are created by scrambling the original input data stream to prevent long runs of 1's or 0's, and encoding the scrambled data using an error correcting code, for example Forward Error Correction (FEC). The coded data is then symbol and frequency interleaved to reduce individual susceptibility to so-called burst errors. The interleaved data stream is then mapped and modulated onto each of the frequency carriers using a suitable modulation technique. Depending on the requirements of the transmission link, different modulation techniques can be used. One such technique is Binary Phase Shift Keying (BPSK).
When using BPSK, the sub-carrier amplitude is nominally set to 1 and the phase of the sub-carrier is set to either 0 or 180 degrees, depending on whether the data the sub-carrier is carrying is a 1 or 0. In such a modulation technique, each sub-carrier carries one bit of information.
The amplitude and phase of the transmitted data are normally expressed as a set of one or more complex numbers in the form (N+iM), where N is the amplitude of the Quadrature part and M is the amplitude of the In-Phase part. The roots of these complex numbers are the points depicted on a Frequency Domain (IQ) diagram, as shown in FIG. 1, which is usually referred to as a constellation in the frequency domain.
The complex numbers that result from BPSK modulation are therefore (1+i0) and (−1+i0) or +1 and −1, that is, the ideal constellation contains just two possible points. This is the case where each bit is encoded to one symbol.
In real WLANs, data is encoded using more complex modulation techniques, such as Quadrature Amplitude Modulation (QAM) or Gaussian Minimum Shift Keying (GMSK), where there are more than two possible root locations, each corresponding to a particular symbol value, and each symbol equating to a set of data bits being sent per transmission time slot, therefore allowing more data to be sent, at the expense of more complex decoding being required.
In systems such as these, where the amplitude and phase of the data can be expressed as a constellation diagram, if an ideal transmitter were to be used, sending its information across an ideal channel, the position of the roots would not change, and therefore the receiver could easily work out what data was being sent by the transmitter, and with no errors. However, real transmitters and channels warp the signal being sent, resulting in roots that are shifted from their ideal positions. The shifts can be seen as rotations about the origin of the IQ axes, or as movement along the length of the axes. These shifts are caused by gain and phase imbalances in the transmit chain, by random phase noise in the transmitter, and by the distortion due to the channel the data is sent over.
In the presence of such shifts, the receiver can incorrectly decode a transmitted root location, because the shifts due to the non-ideal nature of the transmission can result in the roots being moved to (or near) the location of other, equally valid, root locations. These errors are exacerbated in systems with more closely spaced possible root locations, such as QAM. An example of such an erroneous root determination may be understood with reference to FIGS. 2a and 2b. In FIG. 2a, the idealized root of the locations of an arbitrary encoding technique are shown. In FIG. 2b, the actual root locations are shown in solid line of which two are labelled, as 10 and 20. It will be noted that the actual constellation of root locations is rotated about the IQ axes relative to the idealized locations (shown in FIG. 2b in faint). It will also be seen that the actual root locations 10 and 20 are in fact very close to entirely different root locations 30, 40 in the idealized constellation.
The measure of how far a root has moved from its actual, intended location, is called the Error Vector Magnitude (EVM). The measure of this movement as a result of the transmitter only is called the EVM of the transmitter. To allow accurate estimation of the error introduced by the channel, the EVM of the transmitter must be known.
The total EVM of the transmitter is a result of the effects of phase noise, which is random in nature, and from the IQ phase and gain imbalances in the transmit hardware chain, which is systematic in nature.
Since the IQ phase and gain imbalances are systematic in nature, they can be measured, then compensated for in the input signal by pre-distortion of the input signal. The true EVM due to only the random phase noise can then be found. It is this random effect of the phase noise that limits the capability of the transmitter, and thus it is important to test the transmitter for its true EVM to know its limitations or quality. Transmitter Device manufacturers can then remove substandard parts from their production lines.
The procedure for measuring the true EVM of a transmitter that is currently employed first measures the effect of the systematic error introduced by the IQ phase and amplitude imbalances, then pre-distorts the input signal to counteract these errors. The true EVM due to the random phase noise alone is then measured. More specifically, the first test, to measure the IQ phase and amplitude imbalances, involves inputting a single, known frequency and amplitude test tone into both the (and Q inputs. The amplitude and phase offsets are then measured at the output using test equipment. Once these parameters are known, the input signal is then recalculated to pre-distort it to compensate for these errors. The true EVM is then measured in a second test by inputting a test pattern predistorted using the values obtained in the first test, measuring the output and comparing the input to the output. The procedure is thus relatively time-consuming, since it requires two separate tests to be carried out, with a recalculation step in between to take into account the results of the first test. Furthermore, this two step test with a recalculation in between must be carried out for each transmitter separately.